The wide angular range chopper spectrometer (ARCS) is a neutron scattering instrument being developed for the Spallation Neutron Source (SNS). ARCS provides a high neutron flux at the sample, and a large solid angle of detector.
Neutrons at the SNS are produced by the spallation process where high energy protons impact a target producing a large flux of neutrons at many energies in a short period of time. The SNS instruments use time-of-flight (TOF) measurements to determine the energies of the neutrons. To determine the TOF of a detected neutron the detector electronics requires a timing marker that indicates the point in time when protons hit the spallation target.
ARCS is a Fermi chopper spectrometer with a moderate resolution in energy (ω) and a large momentum (Q) transfer range using neutrons with an incident energy of (Ei) from 20 to 2,000 meV. ARCS can be used to advance the science of dynamical processes in materials. It is designed to measure excitations in materials and condensed matter having energies from a few meV to several hundred meV, with an efficiency better than any existing high-energy chopper spectrometer. Applications include, but are not limited to: (i) studies of vibrational excitations and their relationship to phase diagrams and equations of state of materials, including materials with correlated electrons, and (ii) studies of spin correlations in magnets, superconductors, and materials close to metal-insulator transitions.
FIG. 1 shows the schematic diagram of an exemplary ARCS spectrometer 100. The neutrons produced by proton bombardment of the target (not shown) are moderated by a decoupled ambient water moderator 101. Neutrons then propagate through the incident beam line to the sample 110 which is housed in sample chamber 106. Along the incident beam line is a core vessel insert that allows placement of neutron optics close to the moderator 101 and neutron guide from the shutter out.
The beamline of ARCS 100 comprises neutron guide 105 comprising moderator 101, To chopper 102, and Fermi chopper 103. The To chopper 102 is placed about 8.5 m from the moderator 101 to block neutrons when the protons hit the target. The Fermi chopper 103 operates at speeds up to 600 Hz which defines Ei. Detectors 115 detects neutron scattered by sample 110. Detectors 115 are shown disposed in vacuum vessel 120.
The sample 110 and everything beyond it constitute the secondary spectrometer. Neutrons scatter off the sample 110 in the evacuated sample chamber (vessel) 120 and travel to linear position-sensitive detectors (LPSDs) 115 filled with He3 at a pressure of 10 atmospheres (1.0 MPa). The neutron time of flight is measured by these detectors to determine the final energy (Ef) of the scattered neutron.
ARCS is designed to operate with neutrons of 20 meV<Ei<2000 meV. The moderator optimized to provide neutrons in this energy range is the poisoned decoupled water moderator. The poisoning depth is optimized to provide the most flux with a time distribution narrow enough to make a negligible contribution to the instrument's energy resolution. Furthermore, this depth is optimized under the constraint of minimizing the performance degradation to other instruments viewing the same face of this moderator.
Regarding detector 115, the ARCS has been designed to utilize linear position-sensitive detectors. The detector array on ARCS is designed to comprises ˜900 1.0 m long by 25 mm diameter LPSDS. the LPSDS are filled with He3 at a pressure of 10 atmospheres (1.0 mpa). The lengths of the detectors 115 will be divided into pixels of ˜15 mm length by the electronics, for a total of ˜60,000 individual detector elements. Each pixel will subtend an angle of ˜0.5°. Each pixel should have a timing resolution of 1 μs and should saturate at no less than 70,000 n/s. After saturation a tube shall be ready for measurement within 10 μs. The detectors will be grouped into modules of eight within the vacuum vessel. They will feed data to the data acquisition software for manipulation and histogramming, as required.
The detector electronics (not shown in FIG. 1) associated with the detectors 115 are key part of the ARCS. The front end of the detector electronics comprise preamplifier circuitry. Current state of the art pre-amplifier designs utilizes a first operational amplifier having a large feedback resistor (typically >1 Mohms) which is operable to integrate the input charge detected by the detector, the first amplifier being coupled to a pulse shaping filter of the Gaussian type.
ARCS could be simplified by putting the detectors and associate electronics into a vacuum vessel as shown in FIG. 1. However, placing the detector electronics in the vacuum is not compatible with conventional preamplification circuitry due mainly to large amounts of power dissipation. Additionally, other on board support circuitry useful in such environments for example temperature sensors are not in generally used. Moreover, particularly for applications such as ARCS, higher speed electronics that operate well beyond the 100 kHz currently available, as well as improved high voltage protection circuitry and noise rejection, are requirements.